Multi-hop optical communication network

ABSTRACT

An apparatus includes a time-domain wavelength-interleaved optical network that connects a plurality of edge nodes. Each of the edge nodes is configured to receive optical communications from others of the edge nodes on an associated wavelength-channel. The number of edge nodes is larger than the number of the wavelength-channels.

This is a continuation-in-part application of application Ser. No.11/126,024, filed May 10, 2005.

BACKGROUND

1. Field of the Invention

The invention relates generally to optical communication networks, e.g.,time-domain wavelength interleaved networks.

2. Discussion of the Related Art

FIG. 1 schematically shows an exemplary conventional Time-domainWavelength Interleaved Network (TWIN) 10. The TWIN 10 includes a passiveoptical network (PON) 12 that physically interconnects a number of edgenodes 14. The PON 12 has a plurality of internal nodes 16. Each internalnode 16 has an optical cross-connect (OXC) that is configured topassively route optical communications between a plurality of opticaltransmission fibers 18 based solely on wavelength. Each edge node 14includes a wavelength-tunable optical transceiver (not shown). Thus, theedge nodes 14 serve as sources and destinations for opticalcommunications carried between end users by the TWIN.

In the TWIN 10, each wavelength-channel is uniquely assigned to one ofthe edge nodes 14 for receiving optical communications. Then, a sourceedge node 14 transmits an optical communication to a desired edge node14 by simply transmitting the optical communication to the PON 12 on thewavelength-channel that has been assigned to the desired edge node 14.The PON 12 routes the optical communication to a desired destinationedge node 14 based solely on the wavelength of the opticalcommunication. To transmit to a second destination edge node 14, thesource edge node 14 simply resets the transmission wavelength of itsoptical transceiver to the wavelength-channel that has been assigned tothe second destination edge node 14.

Thus, the PON 12 handles routing of optical communicationsautomatically. That is, optical communications do not need label oraddress headers to enable routing. The wavelength alone ensures that oneof the optical communications will be routed to the desired destinationedge node 14.

For that reason, the PON 12 does not have hardware support for bufferingoptical communications at its internal nodes 16. The internal nodes 16automatically and immediately route received optical communicationstowards their destination nodes. Furthermore, the OXCs of the PON 12 cansimultaneously route multiple optical communications so that collisionsbetween different optical communications do not cause information lossat the internal nodes 16 of the PON 12. In particular, a collisionbetween two optical communications will cause a collision at one of thedestination edge nodes 14. Thus, scheduling optical communications tonot have overlapping arrival times at the destination edge nodes 14typically avoids such collisions.

In particular, the PON 12 implements a topology of directed trees inwhich each edge node 14 is a root of a corresponding one of the trees.Each directed tree routes received optical communications via associatedinternal nodes 16 to the tree's root, i.e., the destination edge node14. In such a topology, contention between optical communications occursat the destination edge nodes 14. To reduce such destination contention,source edge nodes 14 schedule transmissions of optical communications asbursts and interleave bursts to different destination edge nodes.

BRIEF SUMMARY

Assigning a reception wavelength-channel to a single edge node limitsthe number of edge nodes to the number of available wavelength-channels.Herein, various embodiments of TWINs assign a receptionwavelength-channel to more than one edge node. Thus, the TWINs have moreedge nodes than the available number of wavelength-channels. Suchconfigurations can enable scaling of the number of edge nodes withoutthe-high expense of increasing the number of availablewavelength-channels.

One embodiment features an apparatus. The apparatus includes atime-domain wavelength-interleaved optical network that connects aplurality of edge nodes. Each of the edge nodes is configured to receiveoptical communications from others of the edge nodes on an associatedwavelength-channel. The number of edge nodes is larger than the numberof the wavelength-channels.

A second embodiment also features an apparatus including an opticalnetwork and a plurality of edge nodes interconnected by the opticalnetwork. Each edge node has an associated wavelength-channel. Theoptical network is configured to route optical communications from somesource ones of the edge nodes to destination ones of the edge nodes inresponse to the communications being on the wavelength-channelsassociated with the destination ones of the edge nodes. The number ofedge nodes is larger than the number of the wavelength-channels.

A third embodiment features a method of transmitting communicationsacross a TWIN. The method includes performing a first routing of acommunication from a source node to a second node via an optical networkof a TWIN based on a carrier wavelength of the communication. The methodincludes determining whether the second node is a destination of thecommunication from routing data carried in the optical communication.The routing data is supplemental to the wavelength-channel of thecommunication in the optical network. The method includes performing asecond routing of the communication from the second node to a third nodevia an optical network of the TWIN in response to determining that thesecond node is not the destination of the communication.

BRIEF DESCRIPTION OF THE DRAWINGS

Various illustrative embodiments are described more fully by the Figuresand below description. Nevertheless, the inventions may be embodied invarious forms and are not limited to embodiments described in theFigures and Detailed Description of Illustrative Embodiments.

FIG. 1 illustrates a conventional time-domain interleaved network(TWIN);

FIG. 2 illustrates an exemplary embodiment of a TWIN;

FIG. 3 illustrates an exemplary configuration of an internal node of theTWIN of FIG. 2;

FIG. 4 illustrates one of the edge nodes of the TWIN of FIG. 2;

FIG. 5 shows the light paths of a specific embodiment of the TWIN shownin FIG. 2;

FIG. 6 shows the virtual topology associated with the specificembodiment of a TWIN as shown in FIG. 5;

FIG. 7 is a flow chart for a method of transmitting communicationsacross a connected TWIN, e.g., the TWIN of FIG. 2;

FIG. 8 illustrates how a color merging process transforms an exemplarytraffic matrix;

FIG. 9 a shows a directed graph that illustrates color-routing in anexemplary TWIN; and

FIG. 9 b illustrates a portion of an auxiliary graph associated with theTWIN of FIG. 9 a.

In the Figures and text, like reference numerals indicate elements withsimilar functions.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

Herein, for simplicity, we sometimes use color to refer to wavelength.Color may refer to wavelengths of visible, infrared, or ultravioletlight.

Referring to FIG. 2, exemplary TWIN 10′ includes a passive opticalnetwork (PON) 12′ and a plurality of edge nodes 14′ that areinterconnected by the PON 12′. The PON 12′ passively routes opticalcommunications between edge nodes 14′ based solely on the wavelengths ofthe optical communications. The PON 12′ includes optical fibertransmission paths 18′ and internal nodes 16′. Each edge node 14′ has amulti-channel optical transceiver for transmitting and receiving opticalcommunications.

FIG. 3 schematically shows how light paths X, Y, and Z are transformedby an exemplary internal node 16′ of the TWIN 10′ shown in FIG. 2. Theexemplary internal node 16′ passively routes light of opticalcommunications between optical ports A, B, C, and D based on wavelength.Each of the optical ports A, B, C, and D connects to paired input andoutput optical fiber transmission paths 18′, which in turn connect toother internal or edge nodes 14′ (not shown). For ones of the opticalports A-D that connect to an edge node 14′, the exemplary internal node16′ functions as an optical add/drop multiplexer (OADM). For ones of theoptical ports A-D that connect to other edge nodes 14′, the exemplaryinternal node 16′ functions as an optical cross-connect (OXC).

The internal node 16′ may merge incoming light paths that carry light ofthe same wavelength, e.g., light paths X and Y. The internal node 16′does not however, split an incoming light path so that received light ofone wavelength is directed to more than one output optical ports in theTWIN 10′. For example, splitting light path Z between light path R andpotential light path S is forbidden.

FIG. 4 schematically shows an exemplary one of the edge nodes 14′ ofFIG. 2. The exemplary edge node 14′ includes a wavelength-tunableoptical transmitter, Tx, a fixed-wavelength optical receiver Rx, and anelectronic switch (SW) that connects the optical receiver Rx to theoptical transmitter Tx and one or more end users (EUs). The opticaltransmitter Tx is tunable to transmit light on the wavelength-channelsassigned to neighboring ones of the edge nodes 14′. Herein, theneighbors of a selected edge node are the other edge nodes that candirectly receive optical communications from the optical transmitter Txof a selected edge node via the PON 12′. The optical receiver Rx isconfigured to receive optical communications at a singlewavelength-channel that is assigned to the associated edge node 14′. Theelectronic switch (SW) electronically routes a received communicationfrom the receiver Rx to either one or more of the local end users (EUs)or to the local transmitter Tx. That is, the electronic switch SWperforms local routing of the communication after conversion fromoptical to electrical form. The electronic switch SW makes this localrouting decision based on supplementary routing data from the receivedcommunication, i.e., data additional to wavelength-channel of thereceived optical communication. The supplementary routing datadetermines whether the local edge node is the final destination or onlyan intermediate edge node for the particular communication.

Referring again to FIG. 2, the PON 12′ passively routes opticalcommunications to edge nodes 14′ based solely on the wavelength-channelsof the optical communications therein. Nevertheless, in the TWIN 10′,multiple edge nodes 14′ are assigned to the same wavelength. That is,the TWIN 10′ has more edge nodes 14′ than available wavelength-channels.For these reasons, one or more of the edge nodes 14′ is not able todirectly communicate with one or more others of the edge nodes 14′solely via optical routing performed by the PON 12′.

Instead, in the TWIN 10′, a source edge node 14′ may only send opticalcommunications directly via the PON 12′ to an associated set of“neighbor” edge nodes 14′. Herein, such direct all-optical transmissionsvia a PON are referred to as single-hop transmissions. The source edgenode 14′ sends communications to the other edge nodes 14′ of the TWINindirectly via one or more intermediate edge nodes 14′, i.e., viamultiple hop transmissions. The intermediate edge nodes 14′ function asdestinations for single-hop optical communications and as gateways formultiple-hop optical communications. As gateways, the intermediate edgenodes 14′ retransmit received optical communications so that suchcommunications can perform one or more other hops through the PON 12′.Such added hops route such communications to edge nodes 14′ that are notneighbors of the original source edge node 14′. During the second orhigher hops, an optical communication may be on the samewavelength-channel as the original optical communication or be on adifferent wavelength-channel. In particular, the wavelength of thecommunication during the second hop depends on the wavelength-assignmentof the second destination edge node 12′ rather than thewavelength-channel of the original optical communication.

In various embodiments of TWINs, a communication may make one, two, ormore hops across the TWIN's PON prior to arriving at its finaldestination edge node. While the TWIN's PON interconnects the edgenodes, the PON itself may or may not be connected.

FIG. 5 shows the light paths associated with single hops for a specificembodiment of the TWIN 10′ shown in FIG. 2. In the specific embodiment,each edge node 14′, i.e., labeled 1, 2, 3, and 4 for convenience,transmits optical communications on two wavelength-channels, i.e.,channels referred to as red and blue, respectively. In this specificembodiment, the edge nodes 1 and 3 are assigned the destination colorblue, and the edge nodes 2 and 4 are assigned the destination color red.For these color assignments, the single-hop light paths for red and bluelight are shown in FIG. 5 with solid and dotted directed lines,respectively. For this set of single-hop light paths, the neighbors ofedge nodes 1, 2, 3, and 4 are given by the respective sets of nodes {3,4}, {3, 4}, {1, 2}, and {2, 3}. Then, two-hop transmissions ofcommunications from edge node 1 to edge node 2, from edge node 2 to edgenode 1, from edge node 3 to edge node 4, and from edge node 4 to edgenode 1, is possible if the intermediate edge nodes are selected from therespective sets {3, 4}, {3}, {1, 2}, and {3}, respectively.

The set of intermediate edge nodes 14′ selectively function as either agateway or a destination in response to receiving an opticalcommunication from the PON 12′. For that reason, these edge nodes 14′support a selection/switching function whose outcome depends on the formof the received optical communication, e.g., the function performed bythe intelligent electronic switch (SW) of FIG. 4. In particular, opticalcommunications carry supplementary routing data that intermediate edgenodes 14′ use to decide whether to operate as a gateway node or as adestination node, i.e., to drop the communication to a local end user.The supplementary routing data is needed, because the wavelength-channelof a communication does not uniquely determine the communication'sdestination when the number of available wavelength-channels is lessthan the number of available destinations.

The optical communications may carry the supplementary routing data as aheader or temporal pulse sequence. The supplementary routing data mayinclude the entire sequence of intermediate edge nodes 14′ to betraversed, i.e., as determined by the original source edge node 14′. Inthe alternative, the supplementary routing data may include only theidentity of the final destination edge node 14′ or the identities of thefinal destination and original source edge nodes 14′. The intermediateedge node 14′ has intelligent circuitry to read the supplementaryrouting data, determine the form of next hop from the data, and senddata representative of the identity of the next receiving edge node 14′to the local optical transmitter. The local optical transmitter uses thedata representative of the identity of the next receiving edge node 14′to reconvert the communication to an optical form on wavelength-channelappropriate for the next hop through the PON 12′.

A TWIN has a virtual topology that is defined by the single-hopstructure of the network. Herein, a virtual topology is a graph ofdirected arcs that is formed on edge nodes of the TWIN. The graphincludes a directed arc from edge node “a” to edge node “b” if and onlyif edge node “a” can transmit an optical communication to edge node “b”via a single hop.

The virtual topology of a conventional TWIN is a fully connecteddirected graph in which a pair of oppositely directed arcs connects eachpair of edge nodes, because any edge node of such a TWIN can opticallytransmit a communication to any other edge node by a single-hop. Incontrast, embodiments of TWINs herein do not have fully connectedvirtual topologies, because they have more edge nodes than the number ofavailable wavelength-channels. Since they have more edge nodes thanwavelength-channels, a given edge node cannot transmit a communicationto, at least, one other edge node via a single hop process. An exampleof a non-fully connected virtual topology is the virtual topology forthe TWIN of FIG. 5, which is shown in FIG. 6.

While the above-description has been limited to particular embodimentsof TWINs, the invention is intended to include other embodiments. Forexample, the PON may have different numbers of internal nodes and/oroptical fiber paths, internal nodes with different numbers of ports,different physical topologies, and different routing configurations atinternal nodes. Also, the edge nodes may have different assignments ofwavelength-channels and/or optical transceivers with different numbersof available wavelength-channels. In addition, the TWINs may beconfigured to implement different virtual topologies.

FIG. 7 illustrates a method 30 for transmitting communications across aconnected network. Examples of the network include TWIN 10′ of FIG. 2and other networks that include a PON and a set of edge nodesinter-connected by the PON, wherein one or more of the edge nodesfunctions as a gateway for the PON.

The method 30 includes performing a first routing of a communication viaan optical network, e.g., of a TWIN, from a source node to a second nodebased solely on the carrier wavelength of the communication (step 32).The method 32 includes determining at the second node whether thedestination node of the communication is the second node (step 34). Thedetermining step includes reading and analyzing routing data carried inthe optical communication. The read routing data is supplementary to thewavelength-channel of the received optical communication in the opticalnetwork. The method 30 includes performing a second routing of thecommunication from the second node to a third node via the opticalnetwork in response to determining that the second node is not thedestination of the communication (step 36). In the second routing step,the optical network routes the communication solely based on its carrierwavelength, which may be the same or different than the carrierwavelength of the optical communication originally received at thesecond node. The step of performing a second routing may includedropping the communication to an edge node that is directly connected tothe second node in response to determining that the second node is thedestination of the communication.

In various embodiments of TWINs, processing of a received communicationby an intermediate edge node may require more time and/or be more costlythan the optical routing of the communication through a single hop overthe PON. Thus, it may be possible to increase the total throughputand/or reduce the total cost by configuring a TWIN to approximatelymaximize single-hop throughputs therein.

Several methods are available to design the configuration of a TWIN in amanner that approximately maximizes single-hop throughputs based on agiven traffic matrix. In a traffic matrix, T, element T_(a,b) definesthe traffic that is carried from source edge node “a” to destinationedge node “b”. The various design methods produce configurations of aTWIN that respect a given physical architecture, i.e., a physicaltopology of a PON and connections between the PON and the edge nodes.The various methods involve performing several steps that configure OXCsand OADMs of internal nodes.

The various methods for designing a TWIN involve performing a colorassignment step (I), a tree design step (II), and a flow routing step(III). The color assignment step involves assigning a destinationwavelength-channel or color to each edge node of the TWIN. The treedesign step involves determining a virtual topology compatible with thegiven physical architecture. In particular, this involves configuringOXCs and OADMs of the PON's internal nodes to define the physical lightpaths therein and fix the virtual topology. The flow routing stepinvolves determining how traffic flows will be routed onto the virtualtopology that is produced by the tree design step. The flow routing stepdetermines how traffic will be distributed among the variousintermediate edge nodes.

I. Color Assignment

The color assignment step assigns W destination wavelength-channels,i.e., colors, to the N edge nodes, wherein W and N are defined by thephysical architecture and N>W. The color assignment is performed in amanner that approximately maximizes the single-hop throughput as definedby the given traffic matrix, T. That is, the color assignment stepapproximately maximizes ${\sum\limits_{({a,b})}T_{a,b}},$wherein the sum is only over node pairs (a, b) that correspond todirected arcs in the virtual topology. The maximization of thesingle-hop throughput is subject to two constraints. The firstconstraint is that each edge node can only optically transmitcommunication traffic to up to W neighbor edge nodes. Thus, in each rowof the traffic matrix, not more than W elements will obtaincontributions from single-hop traffic. The second constraint is that theW neighbor edge nodes of a selected edge node must be assigned differentdestination colors. Thus, by associating destination colors to columnsof the traffic matrix, one sees that only one entry in a row of thetraffic matrix for each column color will obtain a single-hop trafficcontribution.

There are several algorithms for implementing the color assignment step.

In one algorithm, color assignments are produced by a color-mergeprocess. The color-merge process starts by assigning a unique color toeach column of the traffic matrix and then, merges column-colors in apair-wise fashion. Each merger is selected so as to produce the smallestsequential loss of single-hop throughput. A column-color merger causes athroughput loss, because each source edge node can only opticallytransmit to one edge node of a given color. Thus, for each row of thetraffic matrix, a merger of two column-colors requires that one of theelements of the columns, whose colors are merged, obtain a zerosingle-hop traffic contribution. The column-colors to be merged areselected to minimize such losses in single-hop traffic contributions.The merge process is iterated for other pairs of column-colors untilonly W column-colors remain. Then, the remaining colors of the columnsprovide the assignments of destination colors to the associated edgenodes.

FIG. 8 illustrates the evolution of the single-hop contribution to thetraffic matrix as the color merger process progresses for a TWIN having5 edge nodes and three wavelength-channels, i.e., colors. In theprocess, the columns of the traffic matrix, T, are initially assignedthe destination colors: red (R), white (W), blue (B), green (G), andyellow (Y). In the first iteration, the process selects to mergecolumn-colors of columns 4 and 2 so that columns 2 and 4 carry colorlabel, W. This choice of column-color merger leads to the smallestsequential loss of throughput. Merging the colors of columns 2 and 4also requires setting one entry in each row to “0” in a manner thatminimizes the sequential loss of single-hop throughput. In the seconditeration, the process selects to merge column-colors of columns 3 andcolumns 2 and 4 so that columns 2-4 carry color label, W. This choice ofcolumn-color merger leads to the next lowest sequential loss ofthroughput. Merging the colors of columns 2, 3, and 4 again requiressetting one entry in each row to “0” in a manner that minimizes thesequential loss of single-hop throughput. After this second iteration,the process stops, because only three colors remain. From the resultingcolumn-color assignments, the destination color assignments for the edgenodes 1, 2, 3, 4, and 5 are found to be R, W, W, W, and Y, respectively.

In a second color assignment algorithm, an integer linear programmingalgorithm produces the color assignments for the edge nodes. The linearprogramming algorithm uses any conventional method to maximize:$\sum\limits_{({a,b})}{T_{a,b}Q_{a,b}}$subject to the constraints:${{\sum\limits_{k \in W}u_{a}^{k}} = 1},{{\sum\limits_{a \in V}{\gamma^{k}\left( {a,b} \right)}} \leq {Nu}_{b}^{k}},{{\sum\limits_{b \in V}{\gamma^{k}\left( {a,b} \right)}} = 1},\quad{{{and}\quad{\sum\limits_{k \in W}{\gamma^{k}\left( {a,b} \right)}}} = {Q_{ab}.}}$Here, Q is the cross-connect matrix, u^(k) _(b) is a binary variable onthe directed tree of the virtual topology whose root is edge node “b”,γ^(k)(a, b) is a characteristic function for the directed arc of thevirtual topology from edge node “a” to edge node “b”, W is the set ofwavelength-channels, and V is the set of edge nodes of the physical TWINtopology. The matrix element, Q_(,ab) equals “1” if a physical lightpath goes from edge node “a” to edge node “b” and is equal to “0”otherwise. The binary variable u^(k) _(b) is equal to “1” if edge node“b” of the virtual topology is assigned the destination color “k” and isotherwise equal to “0”. The function γ^(k)(a,b) is equal to “1” if thereis a directed arc of the virtual topology from edge node “a” to edgenode “b” is configured to carry color “k” and is otherwise equal to “0”.II. Tree Design

The tree design step involves selecting directed light paths of a givenphysical architecture based on the previous assignment of destinationcolors to the edge nodes. In the directed trees, each destination edgenode is the root of a tree whose leaves are edge nodes that canoptically transmit to the destination edge node. The branches of adirected tree are the light paths through the physical architecture fromthe leaves to the root. Individual branches can merge together as thebranches approach the root. For each color-labeled destination edgenode, the tree design step sequentially builds a color-labeled directedtree by adding physical nodes to the tree whose root is the destinationedge node. The tree design phase tries to find light paths thatimplement a virtual topology that maximizes single-hop throughputs andobeys physical constraints. The physical constraints may prevent thehighest throughput virtual topology from being implemented.

Iterative algorithms are available for forming the directed trees over agiven a physical architecture. In these algorithms, each iterationinvolves selecting a candidate pair of nodes and then, attempting tofind a physical light path that connects the pair of nodes in a mannerconsistent with the existing wavelength-labeled trees. If such a lightpath is found, a directed light path for the pair is added to anexisting tree. When a light path connects edge node “a” to edge node“b”, the process includes adding a directed arc from “a” to “b” in thevirtual topology. The algorithm is iterated until a virtual topology isobtained in which each edge node connects to W outgoing directed arcswhere W is the number of wavelength-channels, i.e., colors.

The algorithm iteratively forms physical light paths by searching forshortest light paths in an auxiliary graph. The light paths are formedby configuring the OXCs and/or OADMs in the internal nodes of the givenPON. During the algorithm, the auxiliary graph enforces OXC/OADMconstraints such that finite-cost light paths of the auxiliary graphcorrespond to light paths compatible with existing wavelength-labeledtrees. The algorithm selects each node for potentially adding to anexisting tree in the order of decreasing traffic matrix elements,T_(a,b). That is, the selection of a potential node to connect to a treeattempts to maximize the sequential increase in single-hop throughput.The algorithm has the following steps.

First, for each color, the algorithm includes setting up a cyclicsequence of physical light paths between the edge nodes of that color.As the cyclic paths are set up, the auxiliary graph is updated to haveappropriate cross-connections for the cyclic paths. For each color, theedge node pairs of that color are now considered both routed and notrejected. The step of setting up such cyclic light paths ensures thatthe final virtual topology will be connected. A simple algorithm isavailable to help in setting up feasible ones of such cyclic sequencesthrough the auxiliary graphs for each of the colors. To use thealgorithm, one considers a directed graph G whose vertices belong to Vand whose directed arcs belong to A such that if arc (i, j) is in A sois arc (j, i). Then, one associates a symmetric positive cost, Cij, toeach directed arc in A. Given U⊂V, the subcycle problem is to find anarc-disjoint cycle in G that covers all of the nodes in U, and theleast-cost subcycle problem is to find the cycle with least-cost. TakingU to be the set of edge nodes of one color, said cycle provides afeasible cycle for connecting said edge nodes. To approach theleast-cost subcyle problem, one forms an m×m matrix {d(i, j)}_(i,j⊂U)where m is the number of elements in U and d(i,j) is the cost of theleast-cost path between nodes “i” and “j” in the graph G. Then, onesolves the traveling salesman problem (TSP) for the matrix {d(i,j}} toobtain a solution to the least-cost subcycle problem. Then, using theshortest paths in G to connect the nodes in U in the order specified bythe solution to the TSP produces a feasible subcycle as desired for theedge nodes of that color. Some methods for heuristically solving the TSPsearch for 2-optimal cycles, i.e., cycles whose length cannot be reducedby inverting a section of the cycle. One way to search for a 2-optimalcycle involves repeatedly searching for favorable inversions of sectionsof a cycle until an improved 2-optimal solution is obtained. Finding a2-optimal solution may provide a low complexity method of finding asolution of subcycle problem even if such a solution is not necessarilyof least-cost.

Next, the algorithm includes repeatedly executing a loop to produce aset of directed trees such that each edge node is the root of one of thedirected trees. The loop includes selecting a remaining edge node pair(i, j) that has the largest traffic matrix element and has not been hasnot been either routed or rejected. Next, the loop includes finding aleast-cost light path of the auxiliary graph from the transmitter at thenode “i” to the receiver at the node “j” where the light path has thewavelength assigned to the destination node “j”. If the cost of thelight path is finite, the light path is kept and a directed arc from “i”to “j” is added in the virtual topology. The loop also includes updatingthe auxiliary graph to show the physical light path that routes the edgenode pair (i, j). If the cost of the light path is infinite, the loopincludes marking the edge node pair (i, j) as rejected. The loop isrepeated for new edge node pairs of the virtual topology until all edgenode pairs are either routed or rejected.

In the above algorithm, using an auxiliary graph enables theconstruction of physical light paths by configuring OXCs and OADMs tomerge common wavelength channels such that the OXCs and OADMs do notsplit an input wavelength channel between multiple output opticalfibers. To construct the auxiliary graph, it is useful to recall thatthe physical topology of the TWIN may be represented by a directedgraph, G(V, A), where V is the set of vertices for the nodes and A isthe set of directed edges for the physical optical fibers that connectthe nodes. Each edge (i, j) ε A is assigned a weight Cij that defines acost for using the edge (i, j). The cost may be a measure of the lengthof the physical link (i, j) or a resource value of the edge, e.g., acost of building and/or operating the associated physical link, whichmay include fiber, repeater, and/or amplifier costs. From the graphG(V,A), an auxiliary graph, G′_(w)(V′, A′) is created for eachwavelength-channel w ε W where W is the set of availablewavelength-channels of the TWIN. In the auxiliary graph, each vertex inV′ represents an input (or output) optical port of a TWIN node onwavelength channel w, or an add or drop port of a TWIN node. The add anddrop ports may represent the transmitter at the source node and receiverat the destination node, respectively.

The transformation for making one of the auxiliary graphs G′_(w) fromthe graph G for the physical topology is described as follows. For eachedge (i, j) ε A where i, j ε V, a corresponding external edge (v_(i)^(out)(j), v_(j) ^(in)(i)) ε A′ is created with a weight Cij where v_(i)^(out)(j) ε V′ denotes the output port at node “i” that is connected tonode j and v_(j) ^(in)(i) ε V′ denotes the input port at node j that isconnected to node i. For each node k ε V, corresponding internal edges(v_(k) ^(in)(i), v_(k) ^(out)(j)) ε A′ are created with a weightC_(k)(i,j) where “i” and “j” are neighboring nodes to node k in G. Theweight C_(k)(i,j) functions as a cost for using the internal edge of thenode “k” that connects the ports for optical fibers connected to nodes“i” and “j”. For each node k connected directly to an edge node, an edge(v_(k) ^(in)(+), v_(k) ^(out)(j)) is created to represent the link fromthe add port (i.e., of the edge node's transmitter) to v_(k) ^(out)(j)for each neighboring node j of node k. Also for each node k connecteddirectly to an edge node, an edge (v_(k) ^(in)(j), v_(k) ^(out)(−)) iscreated to represent the link connecting v_(k) ^(in)(j) to the drop port(i.e., of the edge node's receiver). All internal weights C_(k)(*, *)are initially set to 0.

The sets of weights, i.e., {Cij } and {C_(k)(i,j)}, of an auxiliarygraph are updated as each new path of the graph's color is added to thetree design. The updates enforce physical constraints. For example, if alight path of wavelength w is added and the light path traverses nodesi, k and j in that order, then the update will set all internal weightsof the form C_(k)(i,j′) for j′≠j to infinity in the auxiliary graphG′_(w). Such an update prevents the above-described method from adding asubsequent light path of the same wavelength w with a divergent route atnode “k” thereby avoiding to split an input light path of one wavelengthbetween two different output ports.

The auxiliary graphs can also be used to minimize resource usage whileforming the directed trees of the virtual topology. For example, when anew light path is established to traverse link (i,j) , the cost Cij canbe set to zero to encourage future paths to merge onto the samemultipoint-to-point directed tree.

FIGS. 9 a-9 b illustrate the construction of an auxiliary graph from adirected graph 10″ of an exemplary TWIN.

Referring to FIG. 9 a, the exemplary TWIN has internal nodes 16′,optical fibers 18′, and edge nodes (not shown). In the exemplary TWIN,each internal node connects directly to a corresponding edge node. Theinternal nodes 16′ are enumerated as nodes 1, 2, 3, 4, 5, 6 and alsolabeled by the colors assigned to their corresponding edge nodes, i.e.,R or G. The exemplary TWIN supports optical communications on twowavelength-channels, i.e., red (R) and green (G). The physical lightpaths (LP) for such communications are also shown schematically andlabeled the color, i.e., R or G, of the associated directed tree, i.e.,colors of the destination edge for the light paths.

FIG. 9 b shows a portion 40 of an auxiliary graph for nodes 1 and 2 ofthe exemplary TWIN 10″ shown in FIG. 9 a. The auxiliary graph combinesboth colors for wavelength channels. The portion 40 includes internalnodes 1 and 2 and the corresponding edge nodes 14′, which are set in thenodes 1 and 2 for convenience. The portion 40 includes input and outputports (P), i.e., v^(in,)s and v^(out,)s, and illustrates internal andexternal physical optical fibers (dotted directed lines). The portion 40shows the routing of light paths through the ports (P) and to the edgenodes 14′.

III. Flow Routing

Since the tree design step fixes the virtual topology, i.e., {Q_(a,b)},the flow routing step determines traffic flows over this fixed virtualtopology. In particular, given a source and destination pair, the flowrouting step determines percentages of traffic that will be routedbetween various directed arcs of the virtual topology along paths thatcan transport communications from the given source to the givendestination. Alternatives processes are available for performing theflow routing step.

A first alternative process involves routing flows of communicationsfrom source edge nodes to destination edge nodes via the shortest paths.In this alternative, the flow routings are selected to minimize thenumber of hops needed to go from a source edge node to a destinationedge node.

A second alternative process involves defining the flow routings in amanner that approximately minimizes a congestion measure, C. Thecongestion, C, is taken to be the maximum utilization of any destinationedge node. The minimization is performed subject to constraints on theset of real flow variables, i.e., {F^(s) _(d)(i, j)}, wherein F^(s)_(d)(i, j) is the traffic flow from source node “s” to destination node“d” over the directed arc of the physical topology, V, from node “i” tonode “j”. Here, total flows to each destination are less than or equalto “1”, because the traffic matrix, T, is normalized by the perwavelength data rate. The real flow variables satisfy the followingusual flow constraints:Σ_(i ε V) F ^(s) _(d)(i, k)−Σ_(j ε V) F ^(s) _(d)(k, j)={−T _(s,d) ifk=s,+T _(s,d) if k=d, 0 if k≠d and k≠s}where s, d, i, j, and k are nodes in the physical topology of the TWIN.The real flow variables are also be restricted to be on directed arcs ofthe virtual topology, which imposes the following constraint:Σ_(s,d ε V) F^(s) _(d)(a, b)≦Q_(a,b) for each pair of nodes (a, b)connected by an arc in the virtual topology. Finally, since thecongestion, C, is defined as the maximum utilization of any destinationedge node, the congestion must satisfy Σ_(s, d, i ε V) F^(s) _(d)(i,j)≦C for each physical node j ε V. Typically, a standard multi-commodityflow program can be used to determine the set of real flow variables,which minimize C subject to the above constraints. In addition, suchmethods typically determine the value of C. Provided that the minimumvalue of C is not greater than “1”, the final flow routing design isfeasible over the given physical network. Then, the set of associatedset of real variables {F^(s) _(d)(i, j)} provides the flow informationthat intermediate edge nodes can use to intelligently determine howreceived traffic should be routed.

From the disclosure, drawings, and claims, other embodiments of theinvention will be apparent to those skilled in the art.

1. An apparatus comprising: a time-domain wavelength-interleaved opticalnetwork connecting a plurality of edge nodes, each of the edge nodesbeing configured to receive optical communications from others of theedge nodes on an associated wavelength-channel, the number of edge nodesbeing larger than the number of the wavelength-channels.
 2. Theapparatus of claim 1, wherein at least two of the edge nodes areassigned to the same wavelength-channel.
 3. The apparatus of claim 1,wherein the time-domain wavelength-interleaved optical network comprisesa passive optical network that routes optical communications to the edgenodes based on the wavelength-channels of the optical communications. 4.The apparatus of claim 3, wherein some of nodes of the passive opticalnetwork include optical cross-connects that are configured to passivelyperform routing of optical communications received by the some of thenodes based on the wavelength-channels of the communications.
 5. Theapparatus of claim 1, wherein one of the edge nodes is configured tooptically retransmit some received ones of the optical communications toother edge nodes in response to determining that the some received onesof the optical communications have destinations that are not the one ofthe edge nodes.
 6. The apparatus of claim 1, wherein at least one of theedges nodes is configured to optically retransmit some received ones ofthe optical communications to other ones of the edge nodes on adifferent second wavelength channel.
 7. The apparatus of claim 1,wherein each of the edge nodes is capable of transmitting communicationsto each of the other edge nodes via the TWIN.
 8. An apparatuscomprising: an optical network; and a plurality of edge nodesinterconnected by the optical network, each edge node having anassociated wavelength-channel, the optical network is configured toroute optical communications from some source ones of the edge nodes todestination ones of the edge nodes in response to the communicationsbeing on the wavelength-channels associated with the destination ones ofthe edge nodes, the number of edge nodes is larger than the number ofthe wavelength-channels.
 9. The apparatus of claim 8, wherein at leasttwo of the edge nodes are configured to receive optical communicationson the same wavelength.
 10. The apparatus of claim 8, wherein at leastone of the edge nodes is configured to retransmit some received opticalcommunications to another of the edge nodes on a secondwavelength-channel.
 11. The apparatus of claim 10, wherein the one ofthe edge nodes is configured to determine whether to retransmit areceived ones of the optical communications to the another of the edgenodes based on routing data in the received ones of the communicationthat is supplementary to the wavelength-channels of the ones of theoptical communications.
 12. The apparatus of claim 11, wherein thesupplementary data lists information indicative of the identity of anext intermediate node for the communication.
 13. The apparatus of claim9, wherein the apparatus forms a time-domain interleaved networkingarchitecture.
 14. The apparatus of claim 9, wherein the network hasinternal nodes; each internal node having an optical cross-connect thatis configured to route optical communications between the ports of theassociated internal node based solely on the wavelengths of the opticalcommunications.
 15. A method, comprising: performing a first routing ofa communication from a source node to a second node via an opticalnetwork of a TWIN based on a carrier wavelength of the communication;determining whether the second node is a destination of thecommunication based on supplementary routing data carried in the opticalcommunication; and performing a second routing of the communication fromthe second node to a third node via an optical network of the TWIN inresponse to determining that the second node is not the destination ofthe communication.
 16. The method of claim 15, wherein the performing asecond routing includes transmitting the communication on a differentwavelength-channel than a wavelength-channel used to transmit thecommunication during the performing a first routing.
 17. The method ofclaim 15, wherein each performing step includes transmitting thecommunication to an optical network that passively routes opticalsignals based on wavelength.
 18. The method of claim 15, furthercomprising: determining whether the third node is a destination of thecommunication based on supplementary routing data carried in the opticalcommunication; and performing a third routing of the communication fromthe third node to a fourth node via the optical TWIN in response todetermining that the third node is not the destination of thecommunication.
 19. The method of claim 15, wherein the supplementarydata lists information indicative of the identity of a next intermediatenode for the communication.